Red onion and ricotta frittata

Francesco Mazzei uses finely sliced Tropea red onions and fresh ricotta for his Italian take on the classic frittata. If you are unable to find Tropea red onions he recommends French griotte onions or sweet white onions as a good alternative. This recipe is from Francesco Mazzei's book, Recipes from Southern Italy(Penguin Random House, November 2015).